OA instruments such as personal computers and word processors are now in common use. Various printing apparatuses and methods have thus been developed to print information input via these instruments, on various print media. In particular, owing to their improved information processing capabilities, OA instruments tend to process colored video information. More and more printing apparatuses that output processed information also handle colored images. Various printing apparatuses capable of printing colored images are available and offer various costs and functions. Some printing apparatuses are inexpensive and provide relatively simple functions. Others provide a large number of functions and allow users to select printing speed or image quality depending on the type of images to be printed or the purpose of usage.
Inkjet printing apparatuses can make reduced noise, offer reduced running costs and sizes, and print colored images. Inkjet printing apparatuses are thus widely utilized in printers, copiers, facsimile machines, and the like. In general, color inkjet printing apparatuses print colored images using three color inks, cyan, magenta, and yellow inks, or four color inks, these three inks plus black ink. Conventional inkjet printing apparatuses generally use dedicated paper with an ink absorbing layer as print media in order to print colored images with colors excellently developed, without ink bleeding. Ink is now adapted to suit “ordinary paper”, which is used for printers, copiers, and the like in large quantities.
What is called a serial scan type inkjet printing apparatus uses an inkjet print head in which nozzle groups corresponding to ink colors used for printing are disposed, as printing means for executing color printing using a plurality of color inks. The print head can eject ink from ejection openings constituting the nozzles. The serial scan type inkjet printing apparatus sequentially prints images on print medium by alternately repeating an operation of moving the print head in a main scanning direction, while ejecting ink from the ejection openings in the print head, and an operation of conveying print medium in a sub-scanning direction crossing the main scanning direction. Thus, what is called a horizontal arrangement print head is used in which nozzle groups (groups of nozzles used) corresponding to ink colors used for printing are sequentially horizontally disposed along the main scanning direction. The horizontal arrangement head can eject ink droplets from the nozzle groups onto the same raster during the same printing scan.
To allow the inkjet printing apparatus with the horizontal arrangement head to realize high-resolution printing in order to print images of higher image quality, it is effective to use a high-density print head in which print elements including nozzles are more densely integrated. A high-density print head manufactured by using a semiconductor process has recently emerged. High-density print heads with nozzles formed at 600 dpi (about 42.3 μm) have thus been manufactured.
Moreover, print heads have been manufactured in which a nozzle array corresponding to each ink color is divided into a plurality of parallel nozzle arrays arranged so that the nozzles in one nozzle array are offset from the nozzles in another line by a predetermined amount in the sub-scanning direction. For example, if each nozzle array has a nozzle arrangement density of 600 dpi, two such nozzle arrays are arranged in parallel so that the nozzles in one of the nozzle arrays are offset from the nozzles in the other by 1,200 dpi (about 21.2 μm) in the sub-scanning direction. This results in a print head with a high density of 1,200 dpi.
Another method for printing higher-quality images is a reduction in the size of each ink droplet for image printing. To reduce the size of each droplet, it is effective to use a print head having smaller print elements, including nozzles, able to eject smaller ink droplets. A print head that can eject 4 to 5 pl of ink has recently emerged. Print heads that are advantageous for high-definition printing have thus been manufactured.
Higher-quality images can be printed by thus ejecting smaller ink droplets from densely arranged nozzles.
However, with a horizontal arrangement head, inks ejected from a plurality of nozzle arrays arranged in the main scanning direction may affect one another. Specifically, ink droplets ejected from the nozzles draw in the surrounding air. Thus, when the print head moves at a high speed in the main scanning direction simultaneously with ejection of a large number of ink droplets, an air flow (air current) occurs, which may affect the ejection of the ink.
Now, a specific description will be given of the mechanism of generation of such an air current. First, with reference to FIG. 1, description will be given of how an air current results from operation of a print head.
FIG. 1 is a diagram of an ejection opening formation surface of a print head H as viewed from above. Ejection openings constituting nozzles N are formed in the ejection opening formation surface. Reference characters L1 and L2 denote nozzle arrays from each of which ink is ejected in a direction orthogonal to the sheet of FIG. 1. The print head H executes printing by ejecting ink from the nozzles N in the nozzle arrays L1 and L2 while moving in the main scanning direction, shown by arrow X in FIG. 1. On this occasion, ink droplets ejected vertically below the nozzles N in the nozzle array L1 draw in the surrounding air to form a “gas wall” that moves in the direction of arrow X. The “gas wall” moves in the direction of arrow X to cause air to flow beyond the gas wall to behind it, resulting in an air current flowing in the direction of arrow A in FIG. 1. This air current flows to the front of the nozzle array L2 to affect ink droplets ejected from the nozzles N in the nozzle array L2. The direction of the ejection may thus be shifted.
FIG. 2 is a diagram of the print head H as viewed from its side. This figure shows the flow of air behind the “gas wall”. Ink droplets are ejected from the nozzles N in the nozzle arrays L1 and L2 in the direction of arrow B to cause air to flow downward. The direction of the air flow may change near print medium W so that the air flows rearward as shown by arrow A.
FIG. 3 is a diagram of the print head H as viewed from its front in the main scanning direction; FIG. 3 focuses on the nozzle array L2. In FIG. 3, ink droplets ejected from the nozzles (end nozzles) located at an end of the nozzle array L2 may have their ejecting direction bent toward the longitudinal center of the nozzle array L2 as the ink droplets approach the print medium W, owing to the adverse effect of the air current flowing in the direction of arrow A. If the ejecting direction is bent in that manner, the ink droplets ejected from the end nozzles impact the print medium W at positions that are offset from the original impacting positions toward the longitudinal center of the nozzle array L2. This is recognized as an image defect as is the case with a shift in (bias of) the ejecting direction of ink droplets or non-ejection of ink droplets. The cause of this phenomenon is both the air current flowing to behind the “gas wall”, described with reference to FIG. 1 and the air current resulting from ink ejection as described with reference to FIG. 2; these air currents bend the ejecting direction of ink droplets ejected from the end nozzles.
As described above, a printing apparatus with the conventional horizontal arrangement print head may suffer an image defect caused by air currents resulting from ejection of ink droplets.
Patent Document 1 describes a method used for a multipass printing system of scanning a print head a number of times to complete a predetermined print area; the method controls the amount of ink applied taking into account the relationship between the number of scans (passes) and the adverse effect of air currents. That is, this method controls the amount of ink applied depending on the number of passes in order to avoid the adverse effect of the air currents.
Patent Document 1: European Patent Application Laid-open No. 1405724